September 18, 2011 | 11
For many years I lived in fear of my allergies. While I never had any life-threatening reactions, I developed new allergies with alarming frequency throughout my childhood. First, hives from petting my 4th grade best friend’s bunny, then the standard seasonal allergies of temperate forest springtime, then an increasingly long list of raw fruits, vegetables, and nuts that made my mouth and throat itchy and swollen. I began to think that soon I would be allergic to everything.
A trip to the allergist hoping for some sort of magic or at least a prescription of Zyrtec gave me something better: a diagnosis. I have Oral Allergy Syndrome, a cluster of localized allergies to fresh fruits and vegetables that is pretty common in people with seasonal allergies.
Allergy is your body overreacting to harmless things. The immune system’s antibodies recognize a normally inoffensive protein and activate the immune system to fight it off, causing all of the symptoms you know and hate. For a lot of seasonal allergy sufferers like me, building up antibodies against pollen proteins also means reacting to when that protein shows up in other plants, including fruits and vegetables. Some antibodies bind to the sequence of amino acids in the protein, but many bind to the 3D shape of the protein. Cooking brings the temperature high enough to denature the 3D structure, preventing the antibody from binding and making it edible again.
I’m allergic to birch trees, whose major allergen is called Bet v 1 (Allergen proteins are named based on the Latin name of their host. Birch trees are Betula verrucosa, hence Bet v 1). Bet v 1 is a protein that helps the plant respond to pathogens and shows up in lots of other fruits and vegetables, making me allergic to things like (in rough order of allergenicity): peaches, cherries, almonds, plums, apricots, apples, pears, berries, kiwis, soy, carrots, celery, tomatoes, peppers, parsley, and potatoes.
As the list got longer I assumed I was allergic to something unless proven otherwise. Preferring to err on the side of caution I avoided all uncooked fruits and salads. But more recently, as I started to learn more about plants (woefully lacking from typical molecular biology education) I started to learn the connections between things I was allergic to and things that I could eat. Cucumbers are in the same family as melons! Cherries and almonds are closely related! Broccoli and cabbage are the same species! My allergies started to seem less random, and it became easier to talk with waiters and friends throwing dinner parties about what to avoid.
Mapping fruits and vegetables I can and can’t eat onto the evolutionary tree for flowering plants made the connections even clearer. In the diagram below, modified from a tree constructed by the Angiosperm Phylogeny Group, plant families highlighted in red contain things I’m allergic to, ones in green I can eat, and ones left black are unknown or inedible:
There is a pretty clear split between monocots and eudicots, and the allergenic plants definitely cluster in the rosids and asterales, close to the order Fagales that includes the dreaded birch tree. The clustering makes sense, as proteins should be more closely related in more closely related species, although I’m not really sure how the Cucurbitales, including the non-allergenic cucumbers and melons, can be so close to the mother allergen. It’s also still an open question why only the birch version of this protein that is part of a ubiquitous superfamily present across many different organisms causes this kind of allergic response.
One notable finding from this mapping exercise is that the strength of my allergic reaction seems to correlate with the plant’s position on the phylogenetic tree. From throat-closing peaches and cherries down to mildly irritating (yet delicious) cilantro my allergic reactions do a pretty good job of aligning to phylogenetic relationships. More evolutionary distant versions of Bet v 1 share less and less similarity with the original allergen, so my antibodies bind less well, causing a weaker allergic reaction.
These phylogenetic trees are usually made by comparing the similarity of gene sequences that are shared between all of the members, often a part of the ribosomal RNA. Rebuilding the tree based on Bet v 1 with software that aligns protein sequences:
and clusters them based on relatedness:
yields a very similar tree! These phylogenetic trees now make it easier for me to know if I’m allergic to something and how strong my reaction would be by simply looking up taxonomic information on wikipedia rather than tasting. I still can’t eat any of these things (although my iGEM students did try to engineer non-allergenic plants for me), but thanks to science I’m at least not afraid of my allergies anymore.